Removed AS_USER requirement for SMBulogoff. A valid TID (cnum) is not
given for this call, which become_user needs if it is not to return
an error. This fixes a specific case with NT4.0 which stops users
attaching to a Samba server using a different account to the one they
are logged in under.
(This used to be commit c65a2cdd830bcda396ad6a0bd51a9b80c9d26a67)